1. Introduction

1.1. Purpose of this document

This document is intended to help Linux and Internet users who are
learning by doing. While this is a great way to acquire specific skills,
sometimes it leaves peculiar gaps in one's knowledge of the basics — gaps
which can make it hard to think creatively or troubleshoot effectively,
from lack of a good mental model of what is really going on.

I'll try to describe in clear, simple language how it all works. The
presentation will be tuned for people using Unix or Linux on PC-class
machines. Nevertheless, I'll usually refer simply to ‘Unix’
here, as most of what I will describe is constant across different machines
and across Unix variants.

I'm going to assume you're using an Intel PC. The details differ
slightly if you're running an PowerPC or some other kind of computer, but
the basic concepts are the same.

I won't repeat things, so you'll have to pay attention, but that
also means you'll learn from every word you read. It's a good idea to just
skim when you first read this; you should come back and reread it a few
times after you've digested what you have learned.

This is an evolving document. I intend to keep adding sections in
response to user feedback, so you should come back and review it
periodically.

1.3. Feedback and corrections

If you have questions or comments about this document, please feel
free to mail Eric S. Raymond, at
esr@thyrsus.com. I welcome any suggestions or criticisms. I
especially welcome hyperlinks to more detailed explanations of individual
concepts. If you find a mistake with this document, please let me know so
I can correct it in the next version. Thanks.

1.4. Related resources

If you're reading this in order to learn how to hack, you should also
read the
How To Become A Hacker FAQ. It has links to some other useful
resources.

2. Basic anatomy of your computer

Your computer has a processor chip inside it that does the actual
computing. It has internal memory (what DOS/Windows people call
"RAM" and Unix people often call "core"; the Unix
term is a folk memory from when RAM consisted of ferrite-core donuts). The
processor and memory live on the
motherboard,
which is the heart of your computer.

Your computer has a screen and keyboard. It has hard drives and an
optical CD-ROM (or maybe a DVD drive) and maybe a floppy disk. Some of
these devices are run by controller cards that plug
into the motherboard and help the computer drive them; others are run by
specialized chipsets directly on the motherboard that fulfill the same
function as a controller card. Your keyboard is too simple to need a
separate card; the controller is built into the keyboard chassis
itself.

We'll go into some of the details of how these devices work later. For
now, here are a few basic things to keep in mind about how they work
together:

All the parts of your computer inside the case are connected by a
bus.
Physically, the bus is what you plug your controller cards into (the video
card, the disk controller, a sound card if you have one). The bus is the
data highway between your processor, your screen, your disk, and everything
else.

(If you've seen references to ‘ISA’, ‘PCI’,
and ‘PCMCIA’ in connection with PCs and have not understood
them, these are bus types. ISA is, except in minor details, the same bus
that was used on IBM's original PCs in 1980; it is no longer used.
PCI, for Peripheral Component Interconnection, is the bus used on most
modern PCs, and on modern Macintoshes as well. PCMCIA is a variant of ISA
with smaller physical connectors used on laptop computers.)

The processor, which makes everything else go, can't actually see any of
the other pieces directly; it has to talk to them over the bus. The only
other subsystem that it has really fast, immediate access to is memory (the
core). In order for programs to run, then, they have to be in
core (in memory).

When your computer reads a program or data off the disk, what actually
happens is that the processor uses the bus to send a disk read request
to your disk controller. Some time later the disk controller uses the
bus to signal the processor that it has read the data and put it in a
certain location in memory. The processor can then use the bus to look
at that data.

Your keyboard and screen also communicate with the processor via the
bus, but in simpler ways. We'll discuss those later on. For now, you know
enough to understand what happens when you turn on your computer.

3. What happens when you switch on a computer?

A computer without a program running is just an inert hunk of
electronics. The first thing a computer has to do when it is turned on is
start up a special program called an operating
system. The operating system's job is to help other computer
programs to work by handling the messy details of controlling the
computer's hardware.

The process of bringing up the operating system is called booting (originally this was
bootstrapping and alluded to the process of pulling
yourself up "by your bootstraps"). Your computer knows how to
boot because instructions for booting are built into one of its chips, the
BIOS (or Basic Input/Output System) chip.

The BIOS chip tells it to look in a fixed place, usually on the
lowest-numbered hard disk (the boot disk) for a
special program called a boot loader (under Linux the
boot loader is called Grub or LILO). The boot loader is pulled into memory and
started. The boot loader's job is to start the real operating
system.

The loader does this by looking for a
kernel,
loading it into memory, and starting it. If you Linux and see
"LILO" on the screen followed by a bunch of dots, it is loading the kernel.
(Each dot means it has loaded another disk
block of kernel code.)

(You may wonder why the BIOS doesn't load the kernel directly —
why the two-step process with the boot loader? Well, the BIOS isn't very
smart. In fact it's very stupid, and Linux doesn't use it at all after
boot time. It was originally written for primitive 8-bit PCs with tiny
disks, and literally can't access enough of the disk to load the kernel
directly. The boot loader step also lets you start one of several
operating systems off different places on your disk, in the unlikely event
that Unix isn't good enough for you.)

Once the kernel starts, it has to look around, find the rest of the
hardware, and get ready to run programs. It does this by poking not at
ordinary memory locations but rather at I/O ports
— special bus addresses that are likely to have device controller
cards listening at them for commands. The kernel doesn't poke at random;
it has a lot of built-in knowledge about what it's likely to find where,
and how controllers will respond if they're present. This process is
called
autoprobing.

You may or may not be able to see any of this going on. Back when
Unix systems used text consoles, you'd see boot messages scroll by on your
screen as the system started up. Nowawadays, Unixes often hide the boot
messages behind a graphical splash screen. You may be able to see them by
switching to a text console view with the key combination Ctrl-Shift-F1. If
this works, you should be able to switch back to the graphical boot screen
with a different Ctrl-Shift sequence; try F7, F8, and F9.

Most of the messages emitted boot time are the kernel autoprobing
your hardware through the I/O ports, figuring out what it has available to
it and adapting itself to your machine. The Linux kernel is extremely good
at this, better than most other Unixes and much better
than DOS or Windows. In fact, many Linux old-timers think the cleverness
of Linux's boot-time probes (which made it relatively easy to install) was
a major reason it broke out of the pack of free-Unix experiments to attract
a critical mass of users.

But getting the kernel fully loaded and running isn't the end of the
boot process; it's just the first stage (sometimes called run
level 1). After this first stage, the kernel hands control to a
special process called ‘init’ which spawns several housekeeping
processes. (Some recent Linuxes use a different program called
‘upstart’ that does similar things)

The init process's first job is usually to check to make sure your disks
are OK. Disk file systems are fragile things; if they've been damaged by a
hardware failure or a sudden power outage, there are good reasons to take
recovery steps before your Unix is all the way up. We'll go into some of
this later on when we talk about how file systems can
go wrong.

Init's next step is to start several daemons. A
daemon is a program like a print spooler, a mail listener or a WWW server
that lurks in the background, waiting for things to do. These special
programs often have to coordinate several requests that could conflict.
They are daemons because it's often easier to write one program that runs
constantly and knows about all requests than it would be to try to make
sure that a flock of copies (each processing one request and all running at
the same time) don't step on each other. The particular collection of
daemons your system starts may vary, but will almost always include a print
spooler (a gatekeeper daemon for your printer).

The next step is to prepare for users. Init starts a copy of a
program called getty to watch your screen and keyboard
(and maybe more copies to watch dial-in serial ports). Actually, nowadays
it usually starts multiple copies of getty so you have
several (usually 7 or 8) virtual consoles, with your screen and keyboards
connected to one of them at a time. But you likely won't see any of these,
because one of your consoles will be taken over by the X server (about
which more in a bit).

We're not done yet. The next step is to start up various daemons
that support networking and other services. The most important of these is
your X server. X is a daemon that manages your display, keyboard, and
mouse. Its main job is to produce the color pixel graphics you normally
see on your screen.

When the X server comes up, during the last part of your machine's
boot process, it effectively takes over the hardware from whatever virtual
console was previously in control. That's when you'll see a graphical
login screen, produced for you by a program called a display
manager.

4. What happens when you log in?

When you log in, you identify yourself to the computer. On modern
Unixes you will usually do this through a graphical display manager. But
it's possible to switch virtual consoles with a Ctrl-Shift key sequence and
do a textual login, too. In that case you go through the
getty instance watching that console tto call the
program login.

You identify yourself to the display manager or
login with a login name and password. That login name
is looked up in a file called /etc/passwd, which is a sequence of lines
each describing a user account.

One of these fields is an encrypted version of the account password
(sometimes the encrypted fields are actually kept in a second /etc/shadow
file with tighter permissions; this makes password cracking harder). What
you enter as an account password is encrypted in exactly the same way, and
the login program checks to see if they match. The
security of this method depends on the fact that, while it's easy to go
from your clear password to the encrypted version, the reverse is very
hard. Thus, even if someone can see the encrypted version of your
password, they can't use your account. (It also means that if you forget
your password, there's no way to recover it, only to change it to something
else you choose.)

Once you have successfully logged in, you get all the privileges
associated with the individual account you are using. You may also be
recognized as part of a
group.
A group is a named collection of users set up by the system administrator.
Groups can have privileges independently of their members’ privileges. A
user can be a member of multiple groups. (For details about how Unix
privileges work, see the section below on permissions.)

(Note that although you will normally refer to users and groups by
name, they are actually stored internally as numeric IDs. The password
file maps your account name to a user ID; the
/etc/group
file maps group names to numeric group IDs. Commands that deal with
accounts and groups do the translation automatically.)

Your account entry also contains your home
directory, the place in the Unix file system where
your personal files will live. Finally, your account entry also sets your
shell,
the command interpreter that login will start up to
accept your commmands.

What happens after you have successfully logged in depends on how you
did it. On a text console, login will launch a shell
and you'll be off and running. If you logged in through a display
manager, the X server will bring up your graphical desktop and you will
be able to run programs from it — either through the menus, or
through desktop icons, or through a terminal
emulator running a shell.

5. What happens when you run programs
after boot time?

After boot time and before you run a program, you can think of your
computer as containing a zoo of processes that are all waiting for
something to do. They're all waiting on events. An
event can be you pressing a key or moving a mouse. Or, if your machine is
hooked to a network, an event can be a data packet coming in over that
network.

The kernel is one of these processes. It's a special one, because it
controls when the other user processes can run, and it
is normally the only process with direct access to the machine's hardware.
In fact, user processes have to make requests to the kernel when they want
to get keyboard input, write to your screen, read from or write to disk, or
do just about anything other than crunching bits in memory. These requests
are known as system calls.

Normally all I/O goes through the kernel so it can schedule the
operations and prevent processes from stepping on each other. A few
special user processes are allowed to slide around the kernel, usually by
being given direct access to I/O ports. X servers are the most common
example of this.

You will run programs in one of two ways: through your X server
or through a shell. Often, you'll actually do both, because you'll
start a terminal emulator that mimics an old-fashioned textual console,
giving you a shell to run programs from. I'll describe what happens
when you do that, then I'll return to what happens when you run a program
through an X menu or desktop icon.

The shell is called the shell because it wraps around and hides the
operating system kernel. It's an important feature of Unix that the shell
and kernel are separate programs communicating through a small set of
system calls. This makes it possible for there to be multiple shells,
suiting different tastes in interfaces.

The normal shell gives you the ‘$’ prompt that you see
after logging in (unless you've customized it to be something else). We
won't talk about shell syntax and the easy things you can see on the screen
here; instead we'll take a look behind the scenes at what's happening from
the computer's point of view.

The shell is just a user process, and not a particularly special one.
It waits on your keystrokes, listening (through the kernel) to the keyboard
I/O port. As the kernel sees them, it echoes them to your virtual console or
X terminal emulator. When the kernel sees an ‘Enter’ it passes
your line of text to the shell. The shell tries to interpret those
keystrokes as commands.

Let's say you type ‘ls’ and Enter to invoke the Unix
directory lister. The shell applies its built-in rules to figure out that
you want to run the executable command in the file
/bin/ls. It makes a system call asking the kernel to
start /bin/ls as a new child process and give it
access to the screen and keyboard through the kernel. Then the shell goes
to sleep, waiting for ls to finish.

When /bin/ls is done, it tells the kernel it's
finished by issuing an exit system call. The kernel
then wakes up the shell and tells it it can continue running. The shell
issues another prompt and waits for another line of input.

Other things may be going on while your ‘ls’ is
executing, however (we'll have to suppose that you're listing a very long
directory). You might switch to another virtual console, log in there, and
start a game of Quake, for example. Or, suppose you're hooked up to the
Internet. Your machine might be sending or receiving mail while
/bin/ls runs.

When you're running programs through the X server rather than a shell
(that is, by choosing an application from a pull-down menu, or
double-clicking a desktop icon), any of several programs associated with
your X server can behave like a shell and launch the program. I'm going to
gloss over the details here because they're both variable and unimportant.
The key point is that the X server, unlike a normal shell, doesn't go to
sleep while the client program is running — instead, it sits between
you and the client, passing your mouse clicks and keypresses to it and
fulfilling its requests to point pixels on your display.

6. How do input devices and interrupts work?

Your keyboard is a very simple input device; simple because it
generates small amounts of data very slowly (by a computer's standards).
When you press or release a key, that event is signalled up the keyboard
cable to raise a hardware
interrupt.

It's the operating system's job to watch for such interrupts. For
each possible kind of interrupt, there will be an interrupt
handler, a part of the operating system that stashes
away any data associated with them (like your keypress/keyrelease value)
until it can be processed.

What the interrupt handler for your keyboard actually does is post the
key value into a system area near the bottom of memory. There, it will
be available for inspection when the operating system passes control to
whichever program is currently supposed to be reading from the keyboard.

More complex input devices like disk or network cards work in a similar
way. Earlier, I referred to a disk controller using the bus to signal that
a disk request has been fulfilled. What actually happens is that the disk
raises an interrupt. The disk interrupt handler then copies the retrieved
data into memory, for later use by the program that made the request.

Every kind of interrupt has an associated priority
level.
Lower-priority interrupts (like keyboard events) have to wait on
higher-priority interrupts (like clock ticks or disk events). Unix is
designed to give high priority to the kinds of events that need to be
processed rapidly in order to keep the machine's response smooth.

In your operating system's boot-time messages, you may see references
to IRQ
numbers. You may be aware that one of the common ways to misconfigure
hardware is to have two different devices try to use the same IRQ, without
understanding exactly why.

Here's the answer. IRQ is short for "Interrupt Request". The operating
system needs to know at startup time which numbered interrupts each
hardware device will use, so it can associate the proper handlers with each
one. If two different devices try use the same IRQ, interrupts will
sometimes get dispatched to the wrong handler. This will usually at least
lock up the device, and can sometimes confuse the OS badly enough that it
will flake out or crash.

7. How does my computer do several things at once?

It doesn't, actually. Computers can only do one task (or
process) at a time. But a computer can change tasks
very rapidly, and fool slow human beings into thinking it's doing several
things at once. This is called
timesharing.

One of the kernel's jobs is to manage timesharing. It has a part
called the
scheduler
which keeps information inside itself about all the other (non-kernel)
processes in your zoo. Every 1/60th of a second, a timer goes off in the
kernel, generating a clock interrupt. The scheduler stops whatever process
is currently running, suspends it in place, and hands control to another
process.

1/60th of a second may not sound like a lot of time. But on today's
microprocessors it's enough to run tens of thousands of machine
instructions, which can do a great deal of work. So even if you have many
processes, each one can accomplish quite a bit in each of its
timeslices.

In practice, a program may not get its entire timeslice. If an
interrupt comes in from an I/O device, the kernel effectively stops the
current task, runs the interrupt handler, and then returns to the current
task. A storm of high-priority interrupts can squeeze out normal
processing; this misbehavior is called thrashing and
is fortunately very hard to induce under modern Unixes.

In fact, the speed of programs is only very seldom limited by the
amount of machine time they can get (there are a few exceptions to this
rule, such as sound or 3-D graphics generation). Much more often, delays
are caused when the program has to wait on data from a disk drive or
network connection.

An operating system that can routinely support many simultaneous
processes is called "multitasking". The Unix family of operating
systems was designed from the ground up for multitasking and is very good
at it — much more effective than Windows or the old Mac OS, which both
had multitasking bolted into them as an afterthought and do it rather poorly.
Efficient, reliable multitasking is a large part of what makes Linux
superior for networking, communications, and Web service.

8. How does my computer keep processes from stepping on each other?

The kernel's scheduler takes care of dividing processes in time.
Your operating system also has to divide them in space, so that processes
can't step on each others' working memory. Even if you assume that all
programs are trying to be cooperative, you don't want a bug in one of them
to be able to corrupt others. The things your operating system does to
solve this problem are called memory
management.

Each process in your zoo needs its own area of memory, as a place to
run its code from and keep variables and results in. You can think of this
set as consisting of a read-only code
segment
(containing the process's instructions) and a writeable data
segment
(containing all the process's variable storage). The data segment is truly
unique to each process, but if two processes are running the same code Unix
automatically arranges for them to share a single code segment as an
efficiency measure.

8.1. Virtual memory: the simple version

Efficiency is important, because memory is expensive. Sometimes you
don't have enough to hold the entirety of all the programs the machine is
running, especially if you are using a large program like an X server. To
get around this, Unix uses a technique called virtual memory. It doesn't try to hold all the code and data
for a process in memory. Instead, it keeps around only a relatively small
working set; the rest of the process's state is left in a
special swap space area on your hard disk.

Note that in the past, that "Sometimes" last paragraph ago was
"Almost always" — the size of memory was typically small
relative to the size of running programs, so swapping was frequent. Memory
is far less expensive nowadays and even low-end machines have quite a lot
of it. On modern single-user machines with 64MB of memory and up, it's
possible to run X and a typical mix of jobs without ever swapping after
they're initially loaded into core.

8.2. Virtual memory: the detailed version

Actually, the last section oversimplified things a bit. Yes,
programs see most of your memory as one big flat bank of addresses bigger
than physical memory, and disk swapping is used to maintain that illusion.
But your hardware actually has no fewer than five different kinds of memory
in it, and the differences between them can matter a good deal when
programs have to be tuned for maximum speed. To really understand what
goes on in your machine, you should learn how all of them work.

The five kinds of memory are these: processor registers, internal (or
on-chip) cache, external (or off-chip) cache, main memory, and disk. And
the reason there are so many kinds is simple: speed costs money. I have
listed these kinds of memory in increasing order of access time and
decreasing order of cost. Register memory is the fastest and most
expensive and can be random-accessed about a billion times a second, while
disk is the slowest and cheapest and can do about 100 random accesses a
second.

Here's a full list reflecting early-2000 speeds for a typical desktop
machine. While speed and capacity will go up and prices will drop, you can
expect these ratios to remain fairly constant — and it's those ratios that
shape the memory hierarchy.

Disk

Size: 13000MB Accesses: 100KB/sec

Main memory

Size: 256MB Accesses: 100M/sec

External cache

Size: 512KB Accesses: 250M/sec

Internal Cache

Size: 32KB Accesses: 500M/sec

Processor

Size: 28 bytes Accesses: 1000M/sec

We can't build everything out of the fastest kinds of memory. It
would be way too expensive — and even if it weren't, fast memory is
volatile. That is, it loses its marbles when the power goes off. Thus,
computers have to have hard disks or other kinds of non-volatile storage
that retains data when the power goes off. And there's a huge mismatch
between the speed of processors and the speed of disks. The middle three
levels of the memory hierarchy (internal
cache, external
cache, and main memory) basically exist to bridge
that gap.

Linux and other Unixes have a feature called virtual
memory.
What this means is that the operating system behaves as though it has much
more main memory than it actually does. Your actual physical main memory
behaves like a set of windows or caches on a much larger "virtual" memory
space, most of which at any given time is actually stored on disk in a
special zone called the swap
area. Out of
sight of user programs, the OS is moving blocks of data (called "pages")
between memory and disk to maintain this illusion. The end result is that
your virtual memory is much larger but not too much slower than real
memory.

How much slower virtual memory is than physical depends on how well
the operating system's swapping algorithms match the way your programs use
virtual memory. Fortunately, memory reads and writes that are close
together in time also tend to cluster in memory space. This tendency is
called
locality,
or more formally locality of
reference — and it's a good thing. If memory
references jumped around virtual space at random, you'd typically have to
do a disk read and write for each new reference and virtual memory would be
as slow as a disk. But because programs do actually exhibit strong
locality, your operating system can do relatively few swaps per
reference.

It's been found by experience that the most effective method for a
broad class of memory-usage patterns is very simple; it's called LRU or the
"least recently used" algorithm. The virtual-memory system grabs disk
blocks into its working
set as it
needs them. When it runs out of physical memory for the working set, it
dumps the least-recently-used block. All Unixes, and most other
virtual-memory operating systems, use minor variations on LRU.

Virtual memory is the first link in the bridge between disk and
processor speeds. It's explicitly managed by the OS. But there is still a
major gap between the speed of physical main memory and the speed at which
a processor can access its register memory. The external and internal
caches address this, using a technique similar to virtual memory as I've
described it.

Just as the physical main memory behaves like a set of windows or
caches on the disk's swap area, the external cache acts as windows on main
memory. External cache is faster (250M accesses per sec, rather than 100M)
and smaller. The hardware (specifically, your computer's memory
controller) does the LRU thing in the external cache on blocks of data
fetched from the main memory. For historical reasons, the unit of cache
swapping is called a line rather than a page.

But we're not done. The internal cache gives us the final step-up in
effective speed by caching portions of the external cache. It is faster
and smaller yet — in fact, it lives right on the processor chip.

If you want to make your programs really fast, it's useful to know
these details. Your programs get faster when they have stronger locality,
because that makes the caching work better. The easiest way to make
programs fast is therefore to make them small. If a program isn't slowed
down by lots of disk I/O or waits on network events, it will usually run at
the speed of the smallest cache that it will fit inside.

If you can't make your whole program small, some effort to tune the
speed-critical portions so they have stronger locality can pay off.
Details on techniques for doing such tuning are beyond the scope of this
tutorial; by the time you need them, you'll be intimate enough with some
compiler to figure out many of them yourself.

8.3. The Memory Management Unit

Even when you have enough physical core to avoid swapping, the part
of the operating system called the memory manager
still has important work to do. It has to make sure that programs can only
alter their own data segments — that is, prevent erroneous or malicious
code in one program from garbaging the data in another. To do this, it
keeps a table of data and code segments. The table is updated whenever a
process either requests more memory or releases memory (the latter usually
when it exits).

This table is used to pass commands to a specialized part of the
underlying hardware called an
MMU or
memory management unit. Modern processor chips have MMUs
built right onto them. The MMU has the special ability to put fences
around areas of memory, so an out-of-bound reference will be refused and
cause a special interrupt to be raised.

If you ever see a Unix message that says "Segmentation fault",
"core dumped" or something similar, this is exactly what has happened;
an attempt by the running program to access memory (core) outside its
segment has raised a fatal interrupt. This indicates a bug in the program
code; the core dump it leaves behind is diagnostic information
intended to help a programmer track it down.

There is another aspect to protecting processes from each other besides
segregating the memory they access. You also want to be able to control
their file accesses so a buggy or malicious program can't corrupt critical
pieces of the system. This is why Unix has
file permissions which we'll discuss later.

9. How does my computer store things in memory?

You probably know that everything on a computer is stored as strings of
bits (binary digits; you can think of them as lots of little on-off
switches). Here we'll explain how those bits are used to represent the
letters and numbers that your computer is crunching.

Before we can go into this, you need to understand about the
word size of your computer. The word size is the
computer's preferred size for moving units of information around;
technically it's the width of your processor's
registers,
which are the holding areas your processor uses to do arithmetic and
logical calculations. When people write about computers having bit sizes
(calling them, say, "32-bit" or "64-bit" computers), this is what
they mean.

Most computers now have a word size of 64 bits. In the recent past
(early 2000s) many PCs had 32-bit words. The old 286 machines back in the
1980s had a word size of 16. Old-style mainframes often had 36-bit
words.

The computer views your memory as a sequence of words numbered from
zero up to some large value dependent on your memory size. That value is
limited by your word size, which is why programs on older machines like
286s had to go through painful contortions to address large amounts of
memory. I won't describe them here; they still give older programmers
nightmares.

9.1. Numbers

Integer numbers are represented as either words or pairs of words,
depending on your processor's word size. One 64-bit machine word is the
most common integer representation.

Integer arithmetic is close to but not actually mathematical
base-two. The low-order bit is 1, next 2, then 4 and so forth as in pure
binary. But signed numbers are represented in
twos-complement
notation. The highest-order bit is a sign
bit which
makes the quantity negative, and every negative number can be obtained from
the corresponding positive value by inverting all the bits and adding one.
This is why integers on a 64-bit machine have the range
-263 to 263 - 1.
That 64th bit is being used for sign; 0 means a positive number or zero, 1
a negative number.

Some computer languages give you access to unsigned
arithmetic which is straight base 2 with zero and
positive numbers only.

Most processors and some languages can do operations in
floating-point
numbers (this capability is built into all recent processor chips).
Floating-point numbers give you a much wider range of values than integers
and let you express fractions. The ways in which this is done vary and are
rather too complicated to discuss in detail here, but the general idea is
much like so-called ‘scientific notation’, where one might
write (say) 1.234 * 1023; the encoding of the
number is split into a
mantissa
(1.234) and the exponent part (23) for the power-of-ten multiplier (which
means the number multiplied out would have 20 zeros on it, 23 minus the
three decimal places).

9.2. Characters

Characters are normally represented as strings of seven bits each in
an encoding called ASCII (American Standard Code for Information
Interchange). On modern machines, each of the 128 ASCII characters is the
low seven bits of an
octet
or 8-bit byte; octets are packed into memory words so that (for example) a
six-character string only takes up one 64-bit memory word. For an ASCII code
chart, type ‘man 7 ascii’ at your Unix prompt.

The preceding paragraph was misleading in two ways. The minor one is
that the term ‘octet’ is formally correct but seldom actually
used; most people refer to an octet as
byte
and expect bytes to be eight bits long. Strictly speaking, the term
‘byte’ is more general; there used to be, for example, 36-bit
machines with 9-bit bytes (though there probably never will be
again).

The major one is that not all the world uses ASCII. In fact, much of
the world can't — ASCII, while fine for American English, lacks many
accented and other special characters needed by users of other languages.
Even British English has trouble with the lack of a pound-currency
sign.

There have been several attempts to fix this problem. All use the
extra high bit that ASCII doesn't, making it the low half of a
256-character set. The most widely-used of these is the so-called
‘Latin-1’ character set (more formally called ISO 8859-1).
This is the default character set for Linux, older versions of HTML, and X.
Microsoft Windows uses a mutant version of Latin-1 that adds a bunch of
characters such as right and left double quotes in places proper Latin-1
leaves unassigned for historical reasons (for a scathing account of the
trouble this causes, see the demoroniser
page).

Latin-1 handles western European languages, including English,
French, German, Spanish, Italian, Dutch, Norwegian, Swedish, Danish, and
Icelandic. However, this isn't good enough either, and as a result there
is a whole series of Latin-2 through -9 character sets to handle things
like Greek, Arabic, Hebrew, Esperanto, and Serbo-Croatian. For details,
see the
ISO alphabet soup page.

The ultimate solution is a huge standard called Unicode (and its
identical twin ISO/IEC 10646-1:1993). Unicode is identical to Latin-1 in
its lowest 256 slots. Above these in 16-bit space it includes Greek,
Cyrillic, Armenian, Hebrew, Arabic, Devanagari, Bengali, Gurmukhi,
Gujarati, Oriya, Tamil, Telugu, Kannada, Malayalam, Thai, Lao, Georgian,
Tibetan, Japanese Kana, the complete set of modern Korean Hangul, and a
unified set of Chinese/Japanese/Korean (CJK) ideographs. For details, see
the Unicode Home Page. XML
and XHTML use this character set.

Recent versions of Linux use an encoding of Unicode called UTF-8. In
UTF, characters 0-127 are ASCII. Characters 128-255 are used only in
sequences of 2 through 4 bytes that identify non-ASCII characters.

10. How does my computer store things on disk?

When you look at a hard disk under Unix, you see a tree of named
directories and files. Normally you won't need to look any deeper than
that, but it does become useful to know what's going on underneath if you
have a disk crash and need to try to salvage files. Unfortunately, there's
no good way to describe disk organization from the file level downwards, so
I'll have to describe it from the hardware up.

10.1. Low-level disk and file system structure

The surface area of your disk, where it stores data, is divided up
something like a dartboard — into circular tracks which are then
pie-sliced into sectors. Because tracks near the outer edge have more area
than those close to the spindle at the center of the disk, the outer tracks
have more sector slices in them than the inner ones. Each sector (or
disk block) has the same size, which under modern Unixes
is generally 1 binary K (1024 8-bit bytes). Each disk block has a unique
address or disk block number.

Unix divides the disk into disk
partitions. Each partition is a continuous span of
blocks that's used separately from any other partition, either as a file
system or as swap space. The original reasons for partitions had to do
with crash recovery in a world of much slower and more error-prone disks;
the boundaries between them reduce the fraction of your disk likely to
become inaccessible or corrupted by a random bad spot on the disk.
Nowadays, it's more important that partitions can be declared read-only
(preventing an intruder from modifying critical system files) or shared
over a network through various means we won't discuss here. The
lowest-numbered partition on a disk is often treated specially, as a
boot partition where you can put a kernel to be
booted.

Each partition is either swap
space (used
to implement virtual memory) or a file system used to hold files. Swap-space partitions are
just treated as a linear sequence of blocks. File systems, on the other
hand, need a way to map file names to sequences of disk blocks. Because
files grow, shrink, and change over time, a file's data blocks will not be
a linear sequence but may be scattered all over its partition (from
wherever the operating system can find a free block when it needs
one). This scattering effect is called
fragmentation.

10.2. File names and directories

Within each file system, the mapping from names to blocks is handled
through a structure called an
i-node.
There's a pool of these things near the "bottom"
(lowest-numbered blocks) of each file system (the very lowest ones are used
for housekeeping and labeling purposes we won't describe here). Each
i-node describes one file. File data blocks (including directories) live
above the i-nodes (in higher-numbered blocks).

Every i-node contains a list of the disk block numbers in the file it
describes. (Actually this is a half-truth, only correct for small files,
but the rest of the details aren't important here.) Note that the i-node
does not contain the name of the file.

Names of files live in directory
structures. A directory structure just maps names to
i-node numbers. This is why, in Unix, a file can have multiple true names
(or hard links); they're just multiple directory entries that
happen to point to the same i-node.

10.3. Mount points

In the simplest case, your entire Unix file system lives in just one
disk partition. While you'll see this arrangement on some small personal
Unix systems, it's unusual. More typical is for it to be spread across
several disk partitions, possibly on different physical disks. So, for
example, your system may have one small partition where the kernel lives, a
slightly larger one where OS utilities live, and a much bigger one where
user home directories live.

The only partition you'll have access to immediately after system
boot is your root partition,
which is (almost always) the one you booted from. It holds the root
directory of the file system, the top node from which everything else
hangs.

The other partitions in the system have to be attached to this root
in order for your entire, multiple-partition file system to be accessible.
About midway through the boot process, your Unix will make these non-root
partitions accessible. It will
mount
each one onto a directory on the root partition.

For example, if you have a Unix directory called
/usr, it is probably a mount point to a partition that
contains many programs installed with your Unix but not required during
initial boot.

10.4. How a file gets looked up

Now we can look at the file system from the top down. When you open
a file (such as, say,
/home/esr/WWW/ldp/fundamentals.xml) here is what
happens:

Your kernel starts at the root of your Unix file system (in the root
partition). It looks for a directory there called ‘home’.
Usually ‘home’ is a mount point to a large user partition
elsewhere, so it will go there. In the top-level directory structure of
that user partition, it will look for a entry called ‘esr’ and
extract an i-node number. It will go to that i-node, notice that its
associated file data blocks are a directory structure, and look up
‘WWW’. Extracting that i-node, it will go
to the corresponding subdirectory and look up ‘ldp’. That will
take it to yet another directory i-node. Opening that one, it will find an
i-node number for ‘fundamentals.xml’. That i-node is not a
directory, but instead holds the list of disk blocks associated with the
file.

10.5. File ownership, permissions and security

To keep programs from accidentally or
maliciously stepping on data they shouldn't, Unix has
permission
features. These were originally designed to support timesharing by
protecting multiple users on the same machine from each other, back in the
days when Unix ran mainly on expensive shared minicomputers.

In order to understand file permissions, you need to recall the
description of users and groups in the section
What happens when you log in?. Each file has an owning user and an
owning group. These are initially those of the file's creator; they can be
changed with the programs
chown(1) and
chgrp(1).

The basic permissions that can be associated with a file are
‘read’ (permission to read data from it), ‘write’
(permission to modify it) and ‘execute’ (permission to run it
as a program). Each file has three sets of permissions; one for its owning
user, one for any user in its owning group, and one for everyone else. The
‘privileges’ you get when you log in are just the ability to do
read, write, and execute on those files for which the permission bits match
your user ID or one of the groups you are in, or files that have been made
accessible to the world.

To see how these may interact and how Unix displays them, let's look
at some file listings on a hypothetical Unix system. Here's one:

snark:~$ ls -l notes
-rw-r--r-- 1 esr users 2993 Jun 17 11:00 notes

This is an ordinary data file. The listing tells us that it's owned
by the user ‘esr’ and was created with the owning group
‘users’. Probably the machine we're on puts every ordinary user in
this group by default; other groups you commonly see on timesharing
machines are ‘staff’, ‘admin’, or
‘wheel’ (for obvious reasons, groups are not very important on
single-user workstations or PCs). Your Unix may use a different default
group, perhaps one named after your user ID.

The string ‘-rw-r--r--’ represents the permission bits
for the file. The very first dash is the position for the directory bit;
it would show ‘d’ if the file were a directory, or would show
‘l’ if the file were a symbolic link. After that, the first
three places are user permissions, the second three group permissions, and
the third are permissions for others (often called ‘world’
permissions). On this file, the owning user ‘esr’ may read or
write the file, other people in the ‘users’ group may read it,
and everybody else in the world may read it. This is a pretty typical set
of permissions for an ordinary data file.

Now let's look at a file with very different permissions. This file
is GCC, the GNU C compiler.

This file belongs to a user called ‘root’ and a group
called ‘bin’; it can be written (modified) only by root, but
read or executed by anyone. This is a typical ownership and set of
permissions for a pre-installed system command. The ‘bin’
group exists on some Unixes to group together system commands (the name is
a historical relic, short for ‘binary’). Your Unix might use a
‘root’ group instead (not quite the same as the ‘root'
user!).

The ‘root’ user is the conventional name for numeric user
ID 0, a special, privileged account that can override all privileges. Root
access is useful but dangerous; a typing mistake while you're logged in as
root can clobber critical system files that the same command executed from
an ordinary user account could not touch.

Because the root account is so powerful, access to it should be guarded
very carefully. Your root password is the single most critical piece of
security information on your system, and it is what any crackers and
intruders who ever come after you will be trying to get.

About passwords: Don't write them down — and don't pick a
passwords that can easily be guessed, like the first name of your
girlfriend/boyfriend/spouse. This is an astonishingly common bad practice
that helps crackers no end. In general, don't pick any word in the
dictionary; there are programs called dictionary
crackers that look for likely passwords by running through word
lists of common choices. A good technique is to pick a combination
consisting of a word, a digit, and another word, such as
‘shark6cider’ or ‘jump3joy’; that will make the search
space too large for a dictionary cracker. Don't use these examples, though
— crackers might expect that after reading this document and put them
in their dictionaries.

This file is a directory (note the ‘d’ in the first
permissions slot). We see that it can be written only by esr, but read and
executed by anybody else.

Read permission gives you the ability to list the directory — that
is, to see the names of files and directories it contains. Write permission
gives you the ability to create and delete files in the directory. If you
remember that the directory includes a list of the names of the files and
subdirectories it contains, these rules will make sense.

Execute permission on a directory means you can get through the
directory to open the files and directories below it. In effect, it gives
you permission to access the i-nodes in the directory. A directory with
execute completely turned off would be useless.

Occasionally you'll see a directory that is world-executable but not
world-readable; this means a random user can get to files and directories
beneath it, but only by knowing their exact names (the directory cannot be
listed).

It's important to remember that read, write, or execute permission on a
directory is independent of the permissions on the files and directories
beneath. In particular, write access on a directory means you can
create new files or delete existing files there, but does not
automatically give you write access to existing files.

This has the permissions we'd expect for a system command —
except for that ‘s’ where the owner-execute bit ought to be.
This is the visible manifestation of a special permission called the
‘set-user-id’ or setuid
bit.

The setuid bit is normally attached to programs that need to give
ordinary users the privileges of root, but in a controlled way. When it is
set on an executable program, you get the privileges of the owner of that
program file while the program is running on your behalf, whether or not
they match your own.

Like the root account itself, setuid programs are useful but
dangerous. Anyone who can subvert or modify a setuid program owned by root
can use it to spawn a shell with root privileges. For this reason, opening
a file to write it automatically turns off its setuid bit on most Unixes.
Many attacks on Unix security try to exploit bugs in setuid programs in
order to subvert them. Security-conscious system administrators are
therefore extra-careful about these programs and reluctant to install new
ones.

There are a couple of important details we glossed over when
discussing permissions above; namely, how the owning group and permissions
are assigned when a file or directory is first created. The group is an
issue because users can be members of multiple groups, but one of them
(specified in the user's /etc/passwd entry) is the
user's default group and will normally own files created by the
user.

The story with initial permission bits is a little more complicated.
A program that creates a file will normally specify the permissions it is
to start with. But these will be modified by a variable in the user's
environment called the
umask.
The umask specifies which permission bits to turn off
when creating a file; the most common value, and the default on most
systems, is -------w- or 002, which turns off the world-write bit. See the
documentation of the umask command on your shell's manual page for
details.

Initial directory group is also a bit complicated. On some Unixes a new
directory gets the default group of the creating user (this in the System V
convention); on others, it gets the owning group of the parent directory
in which it's created (this is the BSD convention). On some modern Unixes,
including Linux, the latter behavior can be selected by setting the
set-group-ID on the directory (chmod g+s).

10.6. How things can go wrong

Earlier it was hinted that file systems can be fragile things.
Now we know that to get to a file you have to hopscotch through what may be
an arbitrarily long chain of directory and i-node references. Now suppose
your hard disk develops a bad spot?

If you're lucky, it will only trash some file data. If you're
unlucky, it could corrupt a directory structure or i-node number and leave
an entire subtree of your system hanging in limbo — or, worse, result
in a corrupted structure that points multiple ways at the same disk block
or i-node. Such corruption can be spread by normal file operations,
trashing data that was not in the original bad spot.

Fortunately, this kind of contingency has become quite uncommon as disk
hardware has become more reliable. Still, it means that your Unix will
want to integrity-check the file system periodically to make sure nothing
is amiss. Modern Unixes do a fast integrity check on each partition at
boot time, just before mounting it. Every few reboots they'll do a much
more thorough check that takes a few minutes longer.

If all of this sounds like Unix is terribly complex and
failure-prone, it may be reassuring to know that these boot-time checks
typically catch and correct normal problems before
they become really disastrous. Other operating systems don't have these
facilities, which speeds up booting a bit but can leave you much more
seriously screwed when attempting to recover by hand (and that's assuming
you have a copy of Norton Utilities or whatever in the first
place...).

One of the trends in current Unix designs is journalling
file systems. These arrange traffic to the disk so that
it's guaranteed to be in a consistent state that can be recovered when the
system comes back up. This will speed up the boot-time integrity check a
lot.

11. How do computer languages work?

We've already discussed how programs
are run. Every program ultimately has to execute as a stream of
bytes that are instructions in your computer's machine
language. But human beings don't deal with machine
language very well; doing so has become a rare, black art even among
hackers.

Almost all Unix code except a small amount of direct
hardware-interface support in the kernel itself is nowadays written in a
high-level language. (The ‘high-level’ in this term
is a historical relic meant to distinguish these from
‘low-level’ assembler
languages, which are basically thin wrappers around
machine code.)

There are several different kinds of high-level languages. In order
to talk about these, you'll find it useful to bear in mind that the
source code of a program (the human-created, editable
version) has to go through some kind of translation into machine code that
the machine can actually run.

11.1. Compiled languages

The most conventional kind of language is a compiled
language. Compiled languages get translated into
runnable files of binary machine code by a special program called
(logically enough) a
compiler.
Once the binary has been generated, you can run it directly without looking
at the source code again. (Most software is delivered as compiled binaries
made from code you don't see.)

Compiled languages tend to give excellent performance and have the most
complete access to the OS, but also to be difficult to program in.

C, the language in which Unix itself is written, is by far the most
important of these (with its variant C++). FORTRAN is another compiled
language still used among engineers and scientists but years older and much
more primitive. In the Unix world no other compiled languages are in
mainstream use. Outside it, COBOL is very widely used for financial and
business software.

There used to be many other compiler languages, but most of them have
either gone extinct or are strictly research tools. If you are a new
Unix developer using a compiled language, it is overwhelmingly likely
to be C or C++.

11.2. Interpreted languages

An interpreted
language depends on an interpreter program that reads
the source code and translates it on the fly into computations and system
calls. The source has to be re-interpreted (and the interpreter present)
each time the code is executed.

Interpreted languages tend to be slower than compiled languages, and
often have limited access to the underlying operating system and hardware.
On the other hand, they tend to be easier to program and more forgiving of
coding errors than compiled languages.

Many Unix utilities, including the shell and bc(1) and sed(1) and awk(1),
are effectively small interpreted languages. BASICs are usually
interpreted. So is Tcl. Historically, the most important interpretive
language has been LISP (a major improvement over most of its successors).
Today, Unix shells and the Lisp that lives inside the Emacs editor are
probably the most important pure interpreted languages.

11.3. P-code languages

Since 1990 a kind of hybrid language that uses both compilation and
interpretation has become increasingly important. P-code languages are
like compiled languages in that the source is translated to a compact
binary form which is what you actually execute, but that form is not
machine code. Instead it's
pseudocode
(or
p-code),
which is usually a lot simpler but more powerful than a real machine
language. When you run the program, you interpret the p-code.

P-code can run nearly as fast as a compiled binary (p-code interpreters
can be made quite simple, small and speedy). But p-code languages can keep
the flexibility and power of a good interpreter.

Important p-code languages include Python, Perl, and Java.

12. How does the Internet work?

To help you understand how the Internet works, we'll look at the things
that happen when you do a typical Internet operation — pointing a browser
at the front page of this document at its home on the Web at the Linux
Documentation Project. This document is

which means it lives in the file
HOWTO/Unix-and-Internet-Fundamentals-HOWTO/index.html under the World Wide Web
export directory of the host www.tldp.org.

12.1. Names and locations

The first thing your browser has to do is to establish a network
connection to the machine where the document lives. To do that, it first
has to find the network location of the
host
www.tldp.org (‘host’ is short for ‘host machine’ or ‘network host';
www.tldp.org is a typical
hostname).
The corresponding location is actually a number called an IP
address
(we'll explain the ‘IP’ part of this term later).

To do this, your browser queries a program called a
name server. The name server
may live on your machine, but it's more likely to run on a service machine
that yours talks to. When you sign up with an ISP, part of your setup
procedure will almost certainly involve telling your Internet software the
IP address of a nameserver on the ISP's network.

The name servers on different machines talk to each other, exchanging
and keeping up to date all the information needed to resolve hostnames (map
them to IP addresses). Your nameserver may query three or four different
sites across the network in the process of resolving www.tldp.org, but
this usually happens very quickly (as in less than a second). We'll look
at how nameservers detail in the next section.

The nameserver will tell your browser that www.tldp.org's IP
address is 152.19.254.81; knowing this, your machine will be able to
exchange bits with www.tldp.org directly.

12.2. The Domain Name System

The whole network of programs and databases that cooperates to
translate hostnames to IP addresses is called ‘DNS’ (Domain
Name System). When you see references to a ‘DNS server’, that
means what we just called a nameserver. Now I'll explain how the overall
system works.

Internet hostnames are composed of parts separated by dots. A
domain is a collection of machines that share a common name suffix.
Domains can live inside other domains. For example, the machine
www.tldp.org lives in the .tldp.org subdomain of the .org
domain.

Each domain is defined by an authoritative name
server that knows the IP addresses of the other machines in the
domain. The authoritative (or ‘primary') name server may have backups in
case it goes down; if you see references to a secondary name
server or (‘secondary DNS') it's talking about one of those. These
secondaries typically refresh their information from their primaries every
few hours, so a change made to the hostname-to-IP mapping on the primary
will automatically be propagated.

Now here's the important part. The nameservers for a domain do
not have to know the locations of all the machines in
other domains (including their own subdomains); they only have to know the
location of the nameservers. In our example, the authoritative name server
for the .org domain knows the IP address of the nameserver for
.tldp.org but not the address of all the other
machines in .tldp.org.

The domains in the DNS system are arranged like a big inverted tree.
At the top are the root servers. Everybody knows the IP addresses of the
root servers; they're wired into your DNS software.
The root servers know the IP addresses of the nameservers for the
top-level domains like .com and .org, but not the addresses of machines
inside those domains. Each top-level domain server knows where the
nameservers for the domains directly beneath it are, and so forth.

DNS is carefully designed so that each machine can get away with the
minimum amount of knowledge it needs to have about the shape of the tree,
and local changes to subtrees can be made simply by changing one
authoritative server's database of name-to-IP-address mappings.

When you query for the IP address of www.tldp.org, what actually
happens is this: First, your nameserver asks a root server to tell it where
it can find a nameserver for .org. Once it knows that, it then
asks the .org server to tell it the IP address of a
.tldp.org nameserver. Once it has that, it asks the .tldp.org
nameserver to tell it the address of the host www.tldp.org.

Most of the time, your nameserver doesn't actually have to work that
hard. Nameservers do a lot of cacheing; when yours resolves a hostname, it
keeps the association with the resulting IP address around in memory for a
while. This is why, when you surf to a new website, you'll usually only
see a message from your browser about "Looking up" the host for the first
page you fetch. Eventually the name-to-address mapping expires and your
DNS has to re-query — this is important so you don't have invalid
information hanging around forever when a hostname changes addresses. Your
cached IP address for a site is also thrown out if the host is
unreachable.

12.3. Packets and routers

What the browser wants to do is send a command to the Web server on
www.tldp.org that looks like this:

GET /LDP/HOWTO/Fundamentals.html HTTP/1.0

Here's how that happens. The command is made into a
packet,
a block of bits like a telegram that is wrapped with three important
things; the source address (the IP address of your machine), the
destination address (152.19.254.81), and a service
number
or port number (80, in this case) that indicates that it's a
World Wide Web request.

Your machine then ships the packet down the wire (your connection to
your ISP, or local network) until it gets to a specialized machine called a
router.
The router has a map of the Internet in its memory — not always a complete
one, but one that completely describes your network neighborhood and knows
how to get to the routers for other neighborhoods on the Internet.

Your packet may pass through several routers on the way to its
destination. Routers are smart. They watch how long it takes for other
routers to acknowledge having received a packet. They also use that
information to direct traffic over fast links. They use it to notice when
another router (or a cable) have dropped off the network, and compensate
if possible by finding another route.

There's an urban legend that the Internet was designed to survive
nuclear war. This is not true, but the Internet's design is extremely good
at getting reliable performance out of flaky hardware in an uncertain
world. This is directly due to the fact that its intelligence is
distributed through thousands of routers rather than concentrated in a few
massive and vulnerable switches (like the phone network). This means that
failures tend to be well localized and the network can route around
them.

Once your packet gets to its destination machine, that machine uses the
service number to feed the packet to the web server. The web server can
tell where to reply to by looking at the command packet's source IP
address. When the web server returns this document, it will be broken up
into a number of packets. The size of the packets will vary according to
the transmission media in the network and the type of service.

12.4. TCP and IP

To understand how multiple-packet transmissions are handled, you need to
know that the Internet actually uses two protocols, stacked one on top
of the other.

The lower level,
IP
(Internet Protocol), is responsible for labeling
individual packets with the source address and destination address of two
computers exchanging information over a network.
For example, when you access http://www.tldp.org, the packets you send
will have your computer's IP address, such as 192.168.1.101, and the IP
address of the www.tldp.org computer, 152.2.210.81. These addresses
work in much the same way that your home address works when someone sends
you a letter. The post office can read the address and determine where
you are and how best to route the letter to you, much like a router does
for Internet traffic.

The upper level,
TCP
(Transmission Control Protocol), gives you reliability. When two machines
negotiate a TCP connection (which they do using IP), the receiver knows to
send acknowledgements of the packets it sees back to the sender. If the
sender doesn't see an acknowledgement for a packet within some timeout
period, it resends that packet. Furthermore, the sender gives each TCP
packet a sequence number, which the receiver can use to reassemble packets
in case they show up out of order. (This can easily happen if network
links go up or down during a connection.)

TCP/IP packets also contain a checksum to enable detection of data
corrupted by bad links. (The checksum is computed from the rest of the
packet in such a way that if either the rest of the packet or the
checksum is corrupted, redoing the computation and comparing is very likely
to indicate an error.) So, from the point of view of anyone using TCP/IP
and nameservers, it looks like a reliable way to pass streams of bytes
between hostname/service-number pairs. People who write network protocols
almost never have to think about all the packetizing, packet reassembly,
error checking, checksumming, and retransmission that goes on below that
level.

12.5. HTTP, an application protocol

Now let's get back to our example. Web browsers and servers speak an
application protocol that runs on top of TCP/IP, using it simply
as a way to pass strings of bytes back and forth. This protocol is called
HTTP
(Hyper-Text Transfer Protocol) and we've already seen one command in it —
the GET shown above.

When the GET command goes to www.tldp.org's webserver with service
number 80, it will be dispatched to a server
daemon listening on port 80. Most Internet services
are implemented by server daemons that do nothing but wait on ports,
watching for and executing incoming commands.

If the design of the Internet has one overall rule, it's that all the
parts should be as simple and human-accessible as possible. HTTP, and its
relatives (like the Simple Mail Transfer Protocol,
SMTP,
that is used to move electronic mail between hosts) tend to use simple
printable-text commands that end with a carriage-return/line feed.

This is marginally inefficient; in some circumstances you could get more
speed by using a tightly-coded binary protocol. But experience has shown
that the benefits of having commands be easy for human beings to describe
and understand outweigh any marginal gain in efficiency that you might get
at the cost of making things tricky and opaque.

Therefore, what the server daemon ships back to you via TCP/IP is also
text. The beginning of the response will look something like this (a few
headers have been suppressed):

These headers will be followed by a blank line and the text of the
web page (after which the connection is dropped). Your browser just
displays that page. The headers tell it how (in particular, the
Content-Type header tells it the returned data is really HTML).